Reflection type liquid crystal projector

ABSTRACT

While illuminating light is separated into a first wavelength component illuminating light and second and third wavelength component illuminating lights by a dichroic mirror, the first illuminating light is reflected off a first liquid crystal display element to form a first wavelength component image light which is then reflected off or passed through the polarizing surface of a first polarizing beam splitter, whereas one of the second and third wavelength component illuminating lights has its polarizing surface 90°-converted by a ½ phase difference plate, the other one is passed through the plate without having its polarizing surface converted, the second and third wavelength components are separated from each other by a second polarizing beam splitter and respectively reflected off second and third liquid crystal display elements to obtain second and third wavelength component image lights that are again synthesized, then three wavelength component image lights are synthesized by a dichroic prism, the polarizing surfaces of the first and second polarizing beam splitters used reflect an s polarization light and transmit a p polarization light, and the dichroic prism transmits the second wavelength component image light at a p polarization light and first and third wavelength component image lights at an s polarization light.

FIELD OF THE ART

[0001] This invention relates to a reflection type liquid crystalprojector, and more particularly to an optical system for reflectiontype liquid crystal projector, adapted to project image light of threeprimary colors by splitting uniformly polarized white illuminating lightrays into three RGB wavelength components, producing image light ofthree colors by reflecting the resulting three wavelength componentsrespectively with liquid crystal display panels, and integrating theimage light of the three colors together.

BACKGROUND OF THE ART

[0002] Optical system in existing reflection type liquid crystalprojectors is basically constituted by a color separating optical systemfor splitting illuminating light rays from a white light source intothree RGB wavelength components, three reflection type liquid crystaldisplay panels for reflecting the separated RGB wavelength components ofthe illuminating light to produce image light of the respectivecomponents, and a color integrating optical system for integrating thereflected image light from the respective liquid crystal display panelsinto a full-color image. For example, the color separating opticalsystem is constituted by a combination of optical elements such asdichroic mirror and polarizing beam splitters. On the other hand, thecolor integrating optical system can be constituted by a combination ofoptical elements such as polarizing beam splitter and a dichroic prism.With regard to the optical systems of this sort, various proposal havethus far been made as disclosed, for example, in Laid-Open JapanesePatent Applications 2001-92005, 2001-100155 and H11-326861.

[0003] In the case of the reflection type liquid crystal projectormentioned above, the optical system employs a polarizing beam splitteror splitters as an optical device for separating illuminating light raysinto three RGB wavelength components or for separating two wavelengthcomponents of illuminating light rays after separating one wavelengthcomponent by the use of a dichroic mirror. Further, a polarizing beamsplitter and a dichroic prism are also used at the time of integratingimage light of RGB wavelength components which are reflected by threereflection type liquid crystal display panels.

[0004] In this connection, the polarizing beam splitter and the dichroicprism have been generally considered as an optical device or elementwhich is suitable for transmitting p-polarization light while reflectings-polarization light. Of course, there are optical devices or elementswhich are geared to reflect p-polarization light while transmittings-polarization light. Generally, a polarizing beam splitter or adichroic prism, which has a polarizing film or a dichroic film ofmultiple layer construction, needs to have a more than two times greaternumber of layers to function as an optical element which reflectsp-polarization light and transmits s-polarization light, as comparedwith a polarizing beam splitter or dichroic prism which is designed totransmit p-polarization light and reflect s-polarization light.Naturally, the greater the number of layers, the higher becomes the costas an optical element. Nevertheless, in the optical arrangementaccording to the above-mentioned prior art, a polarizing beam splitterand a dichroic prism are partly used for reflection of p-polarizationlight and transmission of s-polarization light despite a great advantagethat they make the whole optical system extremely expensive.

[0005] Of the above-mentioned prior art publications, Laid-Open JapanesePatent Application H11-326861 discloses a reflection type liquid crystalprojector which is arranged to split illuminating light in the state ofs-polarization light, firstly separating a green wavelength component ofthe illuminating light from blue and red wavelength components by theuse of a dichroic mirror, and then passing the blue and red wavelengthcomponents through a half wave plate thereby to convert the redwavelength component to p-polarization light by rotating the plane ofpolarization by 90 degrees. The green wavelength component of theilluminating light, which is still in the state of s-polarized light, isreflected by one polarizing beam splitter toward a liquid crystaldisplay panel. The p-polarized red component of the illuminating lightand the s-polarized blue component of the illuminating light areseparated by another polarizing beam splitter, which permits thep-polarized red component to pass through a polarizing surface whilereflecting the s-polarized blue component by the polarizing surface. Theseparated red and blue components of the illuminating light are directedtoward the respective liquid crystal display panels. Accordingly, one ofthe polarizing beam splitters in the optical system is required toreflect s-polarization light and transmit p-polarization light.

[0006] In the case of the optical system which is disclosed in Laid-OpenJapanese Patent Application H11-326861, a dichroic prism is used inultimately integrating image light of three wavelength componentstogether. This dichroic prism needs to have characteristics oftransmitting image light of p-polarized green wavelength component whilereflecting image light of s-polarized red wavelength component andp-polarized blue wavelength component.

[0007] In this regard, it is important to note that transmissioncharacteristics or transmittivity of the dichroic prism varyconspicuously depending upon the angle of incidence of input light.Normally, a reflection type liquid crystal projector is provided with aconverging lens in its light source, so that a light flux from the lightsource is converged toward a dichroic prism to enter the latter with acertain angle of incidence. In a case where image light of the greenwavelength component is transmitted through the dichroic prism asp-polarization light and image light of the red wavelength component isreflected by the dichroic prism as p-polarization light as in theabove-described prior art, a conspicuous drop in transmittivity of imagelight of one wavelength component can occur in connection with its angleof incidence to invite degradations in image light output efficiency.This phenomenon makes it necessary to minimize the angle of incidence onthe dichroic prism by using a less bright converging lens with a largerf value, although it will again lead to the problem of degradations inimage light output efficiency.

DISCLOSURE OF THE INVENTION

[0008] In view of the foregoing situations, it is an object of thepresent invention to provide an optical system for a reflection typeliquid crystal projector, which is capable of producing bright andhigh-quality color images by the use of an optical system of simplifiedin construction.

[0009] In order to achieve the above-stated objective, according to thepresent invention, there is provided an optical system for a reflectiontype liquid crystal projector arranged to split a beam of uniformlypolarized white illuminating light, which is projected from a lightsource, into first, second and third wavelength components of threeprimary colors, to reflect said three wavelength components respectivelyby means of liquid crystal display panels to produce image light of thefirst, second and third wavelength components, and to integrate theimage light of the wavelength components together for projecting afull-color image: an optical system which comprises: a dichroic mirroradapted to separate a first wavelength component of the whiteilluminating light from second and third wavelength components; a firstpolarizing beam splitter having a polarizing surface adapted to transmitor reflect the first wavelength component of the illuminating lighttoward a first liquid crystal display panel thereby to reflect imagelight of the first wavelength component; a half-wave plate adapted torotate plane of polarization of one of the second and third wavelengthcomponents through 90 degrees on passage therethrough, without rotatingplane of polarization of the other wavelength component; a secondpolarizing beam splitter having a polarizing surface adapted to reflector transmit the second wavelength component of the illuminating lightfrom the half-wave plate toward a second liquid crystal display panelthereby to reflect image light of the second wavelength component, whiletransmitting or reflecting the third wavelength component of theilluminating light toward a third liquid crystal display panel therebyto reflect image light of the third wavelength component of theilluminating light; and a dichroic prism for integrating together imagelight of the first to third wavelength components coming in from thefirst and second polarizing beam splitters; the polarizing surfaces ofthe first and second polarizing beam splitters being adapted to reflects-polarization light and transmit p-polarization light; and the dichroicprism being adapted to transmit image light of the second wavelengthcomponent in the state of p-polarization light and to reflect imagelight of the first and third wavelength components each in the state ofs-polarization light.

[0010] Namely, the reflection type liquid crystal projector according tothe present invention is basically constituted by a dichroic mirror, acouple of polarizing beam splitters, a half-wave plate and a dichroicprism. In this instance, of the three wavelength components, forexample, if the first wavelength component is a blue component, thesecond wavelength component is a green component and the thirdwavelength component is a red component. Alternatively, if the firstwavelength component is a red component, the second wavelength componentis a green component and the third wavelength component is a bluecomponent. That is to say, the first, second and third wavelengthcomponents are in the order either from the shorter to longer side orfrom the longer to shorter side in wavelength. The dichroic mirrorfunctions to separate the first wavelength component from the second andthird wavelength components. At the half-wave plate, the wavelengthcomponent to be reflected by the second polarizing beam splitter isconverted into s-polarization light, and the wavelength component to betransmitted through the second polarizing beam splitter is convertedinto p-polarization light.

[0011] At the dichroic prism, arrangements are made to suppress to theminimum variations in transmittivity and reflectivity which wouldnormally occur depending upon the angle of incidence of input light. Inthis regard, the optical system employs a dichroic prism which hascharacteristics of reflecting a blue wavelength component in the stateof s-polarization light or characteristics of reflecting a redwavelength component in the state of s-polarization light. In a casewhere the blue component is to be reflected, the red wavelengthcomponent can be transmitted approximately 100% if in the state ofs-polarization light. On the contrary, in a case where the red componentis to be reflected, the blue wavelength component can be transmittedapproximately 100% if in the state of s-polarization light. On the otherhand, with regard to the green wavelength component, even if it isangularly incident on the prism, 100% transmittivity or nearly 100%transmittivity can be obtained as long as it is p-polarization light.However, if the green wavelength component is in the state ofs-polarization light, the transmittivity varies largely depending uponthe angle of incidence. Therefore, the second wavelength component, thatis, the green wavelength component is directed to the dichroic prism asp-polarization light from the second polarizing beam splitter to securehigh transmittivity uninfluenced by the angle of incidence. Further, atthe dichroic prism, one of the blue and red wavelength components whichare both in the state of s-polarization light is reflected by a dichroicsurface and the other one is transmitted through. As a consequence, itbecomes possible to prevent degradations in transmittivity(reflectivity) even when the angle of incidence is increased. In otherwords, it becomes possible for the light source to employ a bright lenswith a small f value as a converging lens.

[0012] Illuminating light may be either p-polarization light ors-polarization light as long as it is uniformly polarized. However, itis most desirable for the dichroic prism to reflect the first wavelengthcomponent in the state of s-polarization light. For this purpose, it isdesirable for the illuminating light to be p-polarization light. In casethe illuminating light is s-polarization light, however, a half-waveplate can be located in front of the first polarizing beam splitterthereby to rotate the plane of polarization of the illuminating lightthrough 90 degrees for conversion into p-polarization light.

[0013] The half-wave plate can be provided independently in the form ofa film which is coated on a plate with flat parallel surfaces. However,it is preferable to coat a retarder film on a light incident surface ofthe second polarizing beam splitter. Further, the first and secondpolarizing beam splitters and the dichroic prism can be provided asseparate optical elements, but it is preferable to integrate theseoptical elements into a single complex structure, eliminating boundarysurfaces in the light paths to the dichroic prism from the first andsecond polarizing beam splitters.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] In the accompanying drawings:

[0015]FIG. 1 is a schematic illustration showing general layout of anoptical arrangement adopted as a first embodiment by the reflection typeliquid crystal projector according to the present invention;

[0016]FIG. 2 is a transmission characteristics diagram of a dichroicmirror;

[0017]FIG. 3 is a transmission characteristics diagram of a polarizingbeam splitter;

[0018]FIG. 4 is a rotative transmission characteristics diagram of ahalf wave plate;

[0019]FIG. 5 is a transmission characteristics diagram of a dichroicprism;

[0020]FIG. 6 is a diagram of angular transmission characteristics withrespect to s-polarized light incident on a dichroic prism suited forreflecting the blue wavelength component;

[0021]FIG. 7 is a diagram of angular transmission characteristics withrespect to p-polarized light incident on a dichroic prism suited forreflecting the blue wavelength component;

[0022]FIG. 8 is an output characteristics diagram of the dichroic prismwith respect to each image light;

[0023]FIG. 9 is a schematic illustration of a modification of theoptical arrangements shown in FIG. 1;

[0024]FIG. 10 is a schematic illustration showing optical glass memberof FIG. 9 in a separated state;

[0025]FIG. 11 is a schematic illustration showing general layout of anoptical arrangement adopted as a second embodiment by the reflectiontype liquid crystal projector according to the present invention;

[0026]FIG. 12 is a schematic illustration of a third embodiment adoptedby the reflection type liquid crystal projector according to the presentinvention;

[0027]FIG. 13 is a schematic illustration of a fourth embodiment adoptedby the reflection type liquid crystal projector according to the presentinvention;

[0028]FIG. 14 is a schematic illustration of a fifth embodiment adoptedby the reflection type liquid crystal projector according to the presentinvention;

[0029]FIG. 15 is a schematic illustration of a sixth embodiment adoptedby the reflection type liquid crystal projector according to the presentinvention; and

[0030]FIG. 16 is a diagram of angular transmission characteristics withrespect to p-polarized light incident on a dichroic prism suited forreflecting the red wavelength component.

BEST MODE FOR CARRYING OUT THE INVENTION

[0031] Referring to FIG. 1, there is shown a first embodiment of thepresent invention, in which indicated at 1 is a white light source andat 2 a polarization converting means, which constitutes an illuminatinglight source section along with the white light source 1. Thepolarization converting means 2 functions to regulate the plane ofpolarization of illuminating light rays from the white light source 1.In the case of the particular embodiment shown, illuminating light isregulated into p-polarized light by the polarization converting means 2.

[0032] Indicated at 3 is a dichroic mirror to which illuminating lightis fed in the first place. This dichroic mirror 3 serves to separate afirst illuminating light component or a blue wavelength component(hereinafter referred to simply as “B component” for brevity) from asecond illuminating light component or a green wavelength component(hereinafter referred to simply as “G component” for brevity) and athird illuminating light component or a red wavelength component(hereinafter referred to simply as “R component” for brevity). Namely,the dichroic mirror 3 functions to transmit B component through whilereflecting G and R components of the illuminating light.

[0033] Denoted at 4 is a polarizing beam splitter (hereinafter referredto simply as “the first PBS” for brevity). The B component of theilluminating light which has been transmitted through the dichroicmirror 3 as p-polarization light is transmitted through a polarizingsurface 4 a of the first PBS 4. Located in a transmitted light path fromthe first PBS 4 is a reflection type liquid crystal display panel 7B(hereinafter referred to simply as “B-LCD” for brevity) for reflectingblue image light. Accordingly, the B component of the illuminating lightis reflected by B-LCD 7B to produce image light of the blue wavelengthcomponent (hereinafter referred to simply as “B image light” forbrevity). From B-LCD 7B, B image light is reflected as s-polarizationlight. This B image light is reflected by the polarizing surface 4 a ofthe first PBS 4 and as a result its light path is bent through 90degrees.

[0034] Indicated at 5 is a half wave plate (hereinafter referred tosimply as “wave plate” for brevity). Of the G and R components of theilluminating light which are reflected from the dichroic mirror 3, thehalf wave plate 5 rotates the plane of polarization of the G componentthrough 90 degrees and transmits same as s-polarization light whiletransmitting the R component on as p-polarization light without rotatingits plane of polarization.

[0035] Indicated at 6 is a second polarizing beam splitter (hereinafterreferred to simply as “the second PBS” for brevity). Of G and Rcomponents of the illuminating light which have been passed through thewave plate 5 as s-polarization light and p-polarization light,respectively, the G component of the illuminating light is reflected bya polarizing surface 6 a of the second PBS 6, while the R component ofthe illuminating light is allowed to pass through. As a result, the Gand R components of the illuminating light are separated from eachother. Located in a light path of the reflected G component is areflection type liquid crystal display panel for green image light 7G(hereinafter referred to simply as “G-LCD 7G” for brevity) thereby toreflect image light of the green wavelength component (hereinafterreferred to simply as “G image light” for brevity). The G component,which is in the state of s-polarization light, is reflected asp-polarization light by G-LCD 7G. On the other hand, the R component,which is p-polarization light, is transmitted through the polarizingsurface 6 a of the second PBS 6 and reflected by a reflection typeliquid crystal display panel R-LCD 7R for red image light (hereinafterreferred to simply as “R-LCD” for brevity) thereby to reflect imagelight of the red wavelength component (hereinafter referred to simply as“R image light” for brevity). The R component is in the state ofp-polarization light, so that the R image light is reflected from R-LCD7R as s-polarization light. Further, the G image light is transmittedthrough the polarizing surface 6 a of the second PBS 6, while the Rimage light is reflected by the polarizing surface 6 a of the second PBS6. As a consequence, the G image light and the R image light areintegrated together.

[0036] Denoted at 8 is a dichroic prism, which serves to output afull-color image by integrating the B image light which has beenreflected by the polarizing surface 4 a of the first PBS 4, with the Gimage light and the R image light which have been integrated together bythe second PBS 6. The dichroic prism 8 is provided with dichroic film 8a with optical characteristics of reflecting the B image light whiletransmitting through the G image light and the R image light. Thus,obtained at the output of the dichroic prism is an integrated colorimage consisting of the three primary colors R, G and B.

[0037] Shown in FIG. 2 is a diagram of spectral characteristics of thedichroic mirror 3. As clear therefrom, the dichroic mirror 3 hascharacteristics of virtually totally transmitting wavelengths smallerthan approximately 500 nm while virtually totally reflecting wavelengthslarger than 500 nm. As a consequence, the B component of shorterwavelength can be separated from the G component of the intermediatewavelength as well as from the R component of longer wavelength. Withregard to optical characteristics of the first PBS 4 and the second PBS6, p-polarization light is almost totally transmitted through asindicated by a solid line in FIG. 3, while s-polarization light isalmost totally reflected as indicated by a broken line. Further, shownin FIG. 4 is a diagram of rotative transmission characteristics of thewave plate 5 with respect to G illuminating light. As clear from thatfigure, rotative transmission by the wave plate is approximately 100% ina wavelength range between 450 nm and 570 nm, and almost 100% of the Gcomponent which is incident on the wave plate 5 as p-polarization lightis converted into s-polarization light upon transmission. The Rcomponent of larger wavelength is transmitted through without rotatingthe plane of polarization. In the particular example shown, the waveplate 5 is arranged to have the rotative transmission characteristics ina narrow wavelength range. However, the wave plate 5 can be arranged ina different way as long as it can convert the G component tos-polarization light and transmit the R component of p-polarizationlight as it is.

[0038] The B image light in s-polarization as well as the G image lightin p-polarization and the R image light in s-polarization are fed to thedichroic prism 8. Of the incident image light, the B image light isreflected by the surface of dichroic film 8 a, while the G image lightand the R image light is transmitted through the dichroic film 8 a.Therefore, the dichroic prism 8 has transmission characteristics asshown in the diagram of FIG. 5. More specifically, in the diagram ofFIG. 5, solid line indicates transmission characteristics with respectto p-polarization light and broken line indicates transmissioncharacteristics with respect to s-polarization light. As clear from thisfigure, the transmission characteristics for p-polarization light is notstable up to a wavelength of approximately 470 nm, but the transmissionis almost at the level of 100% at wavelengths larger than 470 nm. On theother hand, s-polarization light is substantially totally reflected upto a wavelength of approximately 520 nm, and substantially totallytransmitted at larger wavelengths. By the blue reflecting type dichroicprism 8 with the characteristics just mentioned, the B image light ofs-polarization is substantially totally reflected, and the G image lightof p-polarization as well as the R image light of s-polarization issubstantially totally transmitted.

[0039] Although omitted in the drawings, a converging lens is providedin the light source section. Therefore, a light flux from the lightsource is converged and diverged at certain points. Accordingly, a lightbeam which is projected from the light source has a certain angle range.In a case where the f value of the lens is minimized for the sake ofbrightness, the projected light beam becomes to have a greater anglerange, for example, to have an angle range of approximately ±15 degrees.Generally speaking, the transmission and characteristics of dichroiclayers change depending upon the angle of incident light. In the case ofa liquid crystal projector, the optical system is required to havecapability of maximum transmission and reflection to reduce light lossesand to output brighter images.

[0040] In the particular embodiment shown, the dichroic prism 8 isemployed for reflecting the B image light of s-polarization whiletransmitting the G image light of p-polarization and the R image lightof s-polarization. Therefore, the dichroic prism 8 should be of thenature which is suited for reflecting light of wavelengths on theshorter side of the spectral range. Shown in FIGS. 6 and 7 are angularcharacteristics of the dichroic prism of this type. More specifically,shown in FIGS. 6 and 7 are angular characteristics for s-polarizationlight and p-polarization light, respectively. In these figures, a solidline indicates transmission characteristics at the time when the angleof incidence of input light is 0 degree, a broken line indicatestransmission characteristics at the time when the angle of incidence is+15 degrees, and a one-dot chain line indicates transmissioncharacteristics at the time when the angle of incidence is −15 degrees.Accordingly, in the case of FIG. 6, the transmittivity varies dependingupon the angle of incidence in an intermediate wavelength range between500 nm and 560 nm, but the input light is substantially totallyreflected at wavelengths on the shorter and substantially totallytransmitted at wavelengths on the longer side. On the other hand, in thecase of FIG. 7, the transmittivity varies largely depending upon theangle of incidence on the side of short wavelengths, but transmittivityof almost 100% can be obtained in the intermediate wavelength range andon the side of longer wavelengths.

[0041] As described above, the dichroic prism 8 functions to reflect theB image light of s-polarization while transmitting therethrough the Gimage light of p-polarization as well as the R image light ofs-polarization. Therefore, the dichroic prism 8 is completely free fromfactors which would bring about degradations in transmittivity inrelation with the angle of incidence of input light, and, even in a casewhere a bright lens with a small f value is employed as the converginglens, output characteristics of extremely high efficiency can beobtained as shown in FIG. 8 by integrating image of three primary colorsthrough the dichroic prism 8.

[0042] As shown in FIG. 1, the reflection type liquid crystal projectoraccording to the present invention is simple in optical arrangement, andconstituted by a reduced number of optical elements including a dichroicmirror 3, half wave plate 5, a couple of polarizing beam splitters 4 and6 and dichroic prism 8. The simplified construction has an advantagethat it becomes easier to align the optical axes of the respectiveoptical elements. Besides, both of the polarizing beam splitters 4 and6, which function to reflect s-polarization light and transmitp-polarization light, are simplified in polarizing film construction andcan be fabricated at a lower cost. Moreover, the dichroic prism 8 whichfunction to integrate image light of three primary colors has aconspicuous advantage that it is free from degradations intransmittivity dependent on the angle of incidence of input light.

[0043] In this instance, the half wave plate 5 can be provided as anoptical element separate from the polarizing beam splitter 6 if desired.However, it is desirable to form a wave plate film integrally on thelight incident surface of the polarizing beam splitter 6 as shown in thedrawing. Further, the first and second PBSs 4 and 6 can be provided asone optical assembly as shown in FIGS. 9 and 10. Namely, these opticalelements can be integrated into a single optical unit by bonding fouroptical glass elements G1 to G4 together as shown in FIG. 10, formingdichroic or polarizing film layers on necessary surfaces. In the case ofa single optical unit of this sort, it becomes possible to eliminateinterfacial boundaries between the output surfaces of the first andsecond PBSs and the input surfaces of the dichroic prism 8. Besides, thesingle optical unit has further advantages because it becomesunnecessary to make adjustments for alignment of the respective opticalelements or to provide anti-reflection films on boundary surfaces.

[0044] The optical system for reflection type liquid crystal projectoraccording to the present invention can be provided in various forms,other than the one shown in FIG. 1. Some examples of other arrangementsare shown in FIGS. 11 to 15. Firstly, in the case of an optical systemshown in FIG. 11, illuminating light is projected from a light source ina different direction as compared with the optical arrangement shown inFIG. 1. In the case of FIG. 11, of incident illuminating light ofp-polarization, a B illuminating light component is reflected by adichroic mirror 53, while G and R illuminating light components aretransmitted through the dichroic mirror 53. Except for the dichroicmirror 53, other optical arrangements are same as in FIG. 1.

[0045] In the case of an optical system shown in FIG. 12, the opticalsystem is arranged to reflect G and B illuminating light components by adichroic mirror 23 while transmitting an R illuminating light componentthrough the dichroic mirror 23. Accordingly, in this case, R-LCD 7R islocated on the side of the first PBS 4, and B-LCD 7B is located on theside of the second PBS 6. In any case, the same optical elements as inFIG. 1 can be employed, because the first and second PBSs 4 and 6 havecharacteristics of reflecting s-polarization light while transmittingp-polarization light and the wave plate 5 functions to rotate the planeof polarization of the G illuminating light through 90 degrees. However,the dichroic prism 28 is required to reflect the R image light ofs-polarization while transmitting the G image light of p-polarizationand the B image light of s-polarization. Therefore, the dichroic prism28 is adapted to have optical characteristics as shown in FIGS. 16 and17. More specifically, with regard to the transmission characteristicsfor s-polarization light shown in FIG. 16, it is arranged to have hightransmittivity on the side of short wavelengths and high reflectivity onthe side of long wavelengths to preclude the adverse effects of theangle of incidence of input light. With regard to the transmissioncharacteristics for p-polarization light shown in FIG. 17, it hasapproximately 100% transmittivity in short and intermediate wavelengthranges without influenced by the angle of incidence of input light.

[0046] Further, in the case of an optical system shown in FIG. 13, sameoptical arrangements as in FIG. 12 are employed except for the use of adichroic mirror 33 with optical characteristics of reflecting the Rilluminating light component while transmitting the G and B illuminatinglight components.

[0047] Furthermore, in the case of an optical system shown in FIG. 14, abeam of s-polarization light is projected from a light source. In thiscase, the B illuminating light component is transmitted through thedichroic mirror 3 as s-polarization light, so that a half wave plate 49is located in front of input surface of the first PBS 4 thereby torotate the plane of polarization of the B illuminating light through 90degrees before feeding same to the first PBS 4. Another half wave plate45 which is located in front of the input surface of the second PBS 6 inthis case has characteristics of rotating the plane of polarization, notof the G illuminating light component but of the R illuminating lightcomponent through 90 degrees. In the optical system arrangement of FIG.14, it is possible to switch the positions of B-LCD 7B and R-LCD 7R. Insuch a case, the system employs the same dichroic mirror and dichroicprism as in FIG. 12.

[0048] In the case of an optical system shown in FIG. 15, the lightsource is adapted to project s-polarization light in the same manner asin FIG. 14. Dichroic mirror 53 is adapted to reflect the B illuminatinglight component while transmitting the G and R illuminating lightcomponents. That is to say, the dichroic mirror 53 is substantially ofthe same nature as the dichroic mirror 13 in FIG. 11. Similarly to theexample shown in FIG. 14, half wave plates 49 and 45 are positioned infront of the input surfaces of the first and second PBSs 4 and 6,respectively. In the same way as in the above-described optical systemof FIG. 14, it is also possible to switch the positions of B-LCD 7B andR-LCD 7R in the optical system arrangement of FIG. 15.

What is claimed is:
 1. In a reflection type liquid crystal projectorarranged to split a beam of uniformly polarized white illuminatinglight, which is projected from a light source, into first, second andthird wavelength components of three primary colors, to reflect saidthree wavelength components respectively by means of liquid crystaldisplay panels to produce image light of the first, second and thirdwavelength components, and to integrate the image light of saidwavelength components together for projecting a full-color image, anoptical system comprising: a dichroic mirror adapted to separate a firstwavelength component of said white illuminating light from second andthird wavelength components; a first polarizing beam splitter having apolarizing surface adapted to transmit or reflect said first wavelengthcomponent of said illuminating light toward a first liquid crystaldisplay panel thereby to reflect image light of said first wavelengthcomponent; a half-wave plate adapted to rotate plane of polarization ofone of said second and third wavelength components through 90 degrees onpassage therethrough, without rotating plane of polarization of theother wavelength component; a second polarizing beam splitter having apolarizing surface adapted to reflect or transmit said second wavelengthcomponent of the illuminating light from said half-wave plate toward asecond liquid crystal display panel thereby to reflect image light ofsaid second wavelength component, while transmitting or reflecting thethird wavelength component of the illuminating light toward a thirdliquid crystal display panel thereby to reflect image light of saidthird wavelength component of the illuminating light; and a dichroicprism for integrating together image light of the first to thirdwavelength components coming in from said first and second polarizingbeam splitters; said polarizing surfaces of said first and secondpolarizing beam splitters being adapted to reflect s-polarization lightand transmit p-polarization light; and said dichroic prism being adaptedto transmit image light of said second wavelength component in the stateof p-polarization light and to reflect image light of said first andthird wavelength components each in the state of s-polarization light.2. An optical system for a reflection type liquid crystal projector asdefined in claim 1, wherein said illuminating light beam from said lightsource is p-polarization light, said dichroic mirror is adapted totransmit said first wavelength component and to reflect said second andthird wavelength components of the illuminating light, said polarizingsurface of said first polarizing beam splitter is adapted to transmitsaid first wavelength component of the illuminating light toward saidfirst liquid crystal display panel and to reflect said image light ofsaid first wavelength component from said first liquid crystal displaypanel, said half-wave plate is adapted to direct said second and thirdwavelength components of the illuminating light toward said secondpolarizing beam splitter as s-polarization light and p-polarizationlight, respectively, and said polarizing surface of said secondpolarizing beam splitter is adapted to reflect said second wavelengthcomponent of s-polarization light toward said second liquid crystaldisplay panel for reflection by the latter while transmitting said thirdwavelength component of p-polarization light toward said third liquidcrystal display panel for reflection by the latter, and to transmitimage light of said second wavelength component from said second liquidcrystal display panel while reflecting image light of said thirdwavelength component from said third liquid crystal display panel.
 3. Anoptical system for a reflection type liquid crystal projector as definedin claim 2, wherein said first, second and third wavelength componentsof the illuminating light are blue, green and red components,respectively.
 4. An optical system for a reflection type liquid crystalprojector as defined in claim 2, wherein said first, second and thirdwavelength components of the illuminating light are red, green and bluecomponents, respectively.
 5. An optical system for a reflection typeliquid crystal projector as defined in claim 1, wherein saidilluminating light beam from said light source is s-polarization light,said dichroic mirror is adapted to transmit said first wavelengthcomponent and reflect said second and third wavelength components of theilluminating light, another half-wave plate is positioned in front ofsaid first polarizing beam splitter thereby to convert the firstwavelength component into p-polarization light, letting same passthrough said first polarizing beam splitter toward said first liquidcrystal display panel and reflecting the image light of the firstwavelength component from said first liquid crystal display panel bysaid first polarizing beam splitter, said half-wave plate in front ofsaid second polarizing beam splitter is adapted to direct said secondand third wavelength component of the illuminating light toward saidsecond polarizing beam splitter as s-polarization light andp-polarization light, respectively, and said polarizing surface of saidsecond polarizing beam splitter is adapted to reflect wavelengthcomponent of s-polarization light toward said second liquid crystaldisplay panel for reflection by the panel and transmit said thirdwavelength component of p-polarization light through toward said thirdliquid crystal display panel for reflection by the panel, thentransmitting and reflecting image light of said second and thirdwavelength components reflected from said second and third liquidcrystal display panels, respectively.
 6. An optical system for areflection type liquid crystal projector as defined in claim 1, whereinsaid half-wave plate is provided as a film formed on a light incidentsurface of said second polarizing beam splitter.
 7. An optical systemfor a reflection type liquid crystal projector as defined in claim 1,wherein optical elements of said first and second polarizing beamsplitters and said dichroic prism are bonded together and integratedinto a unitary optical structure in such a way as to eliminate boundarysurfaces between said optical elements.